1. Field of the Invention
The present invention relates to a head of a golf club and, more particularly, to an improvement of a head of a golf club having therein a through hole for allowing air to pass therethrough during a swing of the club, to reduce aerodynamic resistance acting on the head during the swing.
2. Description of the Related Art
When a conventional head is swung at an ordinary kinetic speed, an air flow boundary layer in form of a laminar air flow boundary layer and/or a turbulent air flow boundary layer, i.e., an intermediate boundary layer at the point of transition from a laminar air flow to a turbulent air flow, is produced on the outer surface of the head within a region between the face side and the back side of the head. The air flow boundary layer is exfoliated at the rear end thereof from the outer surface of the head, and thus a subatmospheric pressure region is created behind the head. Therefore, a turbulent air flow or swirling air stream is produced behind the head by the subatmospheric pressure.
In this air stream condition around the head during a swing, air in front of the head is compressed and the air pressure increased, but the air pressure behind the head is reduced. Accordingly, an aerodynamic drag due to the pressure difference in front of and behind the head (hereinafter referred to as "pressure drag") is imposed on the head during the swing. It is known that such a pressure drag reduces the head speed during the swing, and thus reduces a distance of flight of a ball hit by the head. It is also known that such a pressure drag has an adverse affect on the stability of the movement of the head during the swing.
Japanese Unexamined Utility Model Publication Nos. 48-60672, 50-85367, 51-116060, and 59-33459, and Japanese Examined Utility Model Publication No. 50-24276 discloses a conventional head of a golf club in which a head body having a face side for hitting a golf ball is formed with at least one through hole allowing air to pass therethrough, for reducing air resistance or wind pressure acting on the head body during a swing. In this type of head, the through hole is open at the front and rear ends thereof in the face and rear sides of the head, respectively, so that an air pressure difference between the front of and behind the head, produced during a swing of the head, can be reduced.
However, in the conventional head having the above-mentioned through hole, since the front end of the through hole is open in the face side of the head for hitting a golf ball, at the center of the face side or in the vicinity of the toe or heel side of the head, the head tends to hit a golf ball at an area in which the front end of the through hole is open. Accordingly, when a ball is hit by a head having a through hole at an area in which the front end of the through hole is open, an even stress is produced in the ball, and thus the direction of flight of the ball hit by the head is deteriorated. Particularly, when a front end of a through hole is open in a face side of the head in the vicinity of the toe or heel side of the head, and a golf ball is hit by the head at a region in which a through hole is open, it becomes difficult to produce side spin on the ball due to a reduction of the area of the head in contact with the ball. As the result, the direction of flight of a ball hit by the head is deteriorated.